Can you start a heli's blades mid-air?  
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Can you start a heli's blades mid-air? - 9/8/2005 4:46:41 AM   
Gravityisnotmyfriend



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Status: offline 		I was watching the most recent bond movie. I just caught the end and am not sure which one it is...it's the one with Halle Berry. Anyway, in their big escape scene, they ride a heli as it slides out of a cargo plane, start the engine and rotor as they fall. Of course, they get it flying just before impact and fly away safely. I was just wondering if this is possible. I'm not worried about how long it takes to start the turbine in the heli, I'm just wondering if it is possible to spin the rotors up to speed while the heli is falling at terminal velocity. I'm thinking that it is not, but I want to hear some expert opinions.

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RE: Can you start a heli's blades mid-air? - 9/8/2005 1:59:19 PM   
BarracudaHockey



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Status: offline 		Picture rotor blades bent straight up.

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RE: Can you start a heli's blades mid-air? - 9/8/2005 3:57:18 PM   
Gravityisnotmyfriend



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Joined: 3/1/2004
From: Greenville, WI, USA
Status: offline 		I see your point. I've seen that most full scale heli's rotor blades droop a little when it is sitting. So, without the centrifugal forces holding the blades out, they would fail under the weight of the heli. Thanks.



-edit for spelling

< Message edited by Gravityisnotmyfriend -- 9/8/2005 3:58:47 PM >

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The mathematician sees the glass as twice as big as it needs to be. The engineer sees it as full with a Safety Factor of 2.

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RE: Can you start a heli's blades mid-air? - 9/8/2005 4:10:51 PM   
BarracudaHockey



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Status: offline 		It's the same priciple as a long thin sailplane wing, as long as it is flying through the air its fine. Force it straight down at terminal velocity and SNAP

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RE: Can you start a heli's blades mid-air? - 9/8/2005 4:24:43 PM   
Gravityisnotmyfriend



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Status: offline 		OK, so the situation shown the movie couldn't have happened. But, what if the heli was dropped with full negative collective. Could the rotors spin up in aoutrotation before they snapped? It's kind of a pointless question - I'm just wondering if it's possible.

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RE: Can you start a heli's blades mid-air? - 9/8/2005 7:29:50 PM   
BarracudaHockey



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Status: offline 		I doubt it, I don't think it would ever obtain a stable fall but I do know one thing for sure.


I wouldn't want to be the one to try it.

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RE: Can you start a heli's blades mid-air? - 9/8/2005 8:05:34 PM   
Flyfalcons



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Status: online 		RC helis have done full blade stop autos. The trick is to get them spinning again before the heli starts tumbling out of control. Not something that I'm going to try with anything I own unless people start paying for my helis.

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RE: Can you start a heli's blades mid-air? - 9/8/2005 9:04:12 PM   
BarracudaHockey



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Status: offline 		RC Heli blades are a lot shorter and stiffer than real heli blades.

I agree, no blade stop autos for me (on purpose anyway)

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RE: Can you start a heli's blades mid-air? - 9/8/2005 11:55:35 PM   
Gravityisnotmyfriend



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Status: offline 		I've done full blade stop autos on an RC heli ..........on my G2 But since my success rate is only about 10%, Im not gonna try it with my heli.

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RE: Can you start a heli's blades mid-air? - 9/9/2005 5:00:48 AM   
Pahtcub


 

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Status: offline 		Somewhere I saw where they used rockets on the ends of the rotors to spin helicopter blades to speed quickly, can't remember if it was a movie or documentary, was a few years ago.

Pat

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RE: Can you start a heli's blades mid-air? - 9/23/2005 3:18:53 AM   
William Robison



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From: Mary Esther, Florida, FL, USA
Status: offline 		Pat:

You might be thinking about the Hiller "Hornet." It had a ram jet engine at each rotor tip, and a hand crank in the cockpit for the initial spin of the rotor - necessary to get the air flow for the RJ engines to start.

Unsuccessful for several reasons. Not the least was the noise, also sealing problems feeding the fuel through the rotor hub, Tou also needed Uncle Sam paying your fuel bill, ram jets are anything but economical.

But an air start? Doubt it.

A couple interesting pages on the H-23 Hornet:

http://www.fantasyofflight.com/aircraftpages/hornet.htm
http://kapowiezone.blogspot.com/2004/08/hiller-hornet-coupe.html

Bill.

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RE: Can you start a heli's blades mid-air? - 10/18/2005 9:52:23 AM   
DT56


 

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Status: offline 		The reason this wouldn't work is well known to full scale pilots. Full scale helicopters have a very limited range of collective pitch travel compared to RC models. There are definite high and low RPM limits for the rotor system, with the low end being the greatest concern. Lower rotor RPM causes the aircraft to descend due to loss of lift. Adding collective pitch to compensate causes an increase in drag at the blade, further decreasing RPM. Once reaching a certain point, the blade stalls and drag exceeds the power available from the engine, with RPM decreasing even further. There is insufficient negative collective pitch to allow a recovery of RPM and the rotor speed continues to decay until the aircraft augers in. Since helicopters are usually flown fairly low, most crash before RPM reduces to the point of blade structural failure.
In a helicopter with a low inertia rotor system, such as the Robinson R22, there are flight conditions in which full down collective pitch is required within 2 seconds of an engine failure to prevent reaching an unrecoverable low rotor RPM. No surprise that maintaining rotor RPM is the most important lesson learned in flight training!

DT56


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RE: Can you start a heli's blades mid-air? - 11/8/2005 5:11:31 AM   
Skiddz


 

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Status: offline 		You've got to be real quick to enter an auto in the R22 if you want to live to talk about it. Frank Robinson says you've got about 1.1 seconds to react before low rotor rpm kills you. The R44 is an entirely different beast. Autos are a non-event for all practical purposes.. When I got my checkout in the Raven II, I had to do a full down auto, pick up and do a 180 pedal turn and set it down again. PLENTY of inertia to do so..

As for starting blades in midair? No way. They'd tulip up on you in a heartbeat. No centrifugal force to keep 'em from doing so.. Now if they made real helo blades as stiff as our models.. Possible.

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RE: Can you start a heli's blades mid-air? - 12/15/2005 6:30:21 AM   
BigWes



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Joined: 11/1/2005
From: Opp, AL, USA
Status: offline 		Nice thread here. I am a real life helicopter mech. No that cant happen what they did in that movie. In a real heli centrifigal force is what keeps the blades from snapping. I have seen seven broken blades from real life helicopters that were broken from nothing more than high winds on the ground. If the blades flex with enough pressure they will snap and peel apart. Blades have a main spar with honeycomb filler and skin over the top. Bell helicopter manual will give limits on some of their helicopter blades. Take the 206B for instance. The maint manual says any more than 100 pounds of force in any direction up or down on a stationary blade may cause failure. Helicopters are scientific marvels and maintenance nightmares. I like'em anyway I don't fly r/c helis. I wish I could at least try one some time but I don't have the money for them. Just remember there is a lot of stuff on tv that is nothing but crap. If you don't believe that you may wanna watch an old episode of star trek LOL

Wes

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RE: Can you start a heli's blades mid-air? - 12/12/2006 2:02:01 AM   
47steve


 

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From: enterprise, AL, USA
Status: offline 		As far as re-starting inflight, it can be done. As stated above, rotor RPM is the single most crucial instrument to watch when doing autos. If rotor RPM goes too high, you overspeed and if it goes down, it underspeeds. In an emergency, you are more concerned with underspeeding as a stationary rotor renders your helicopter useless and you just fall out of the sky. Rotor overspeed...keep it below limits as it will easily overspeed, but is easier to correct with a little collective application, whereas underspeed, you drop the collective smoothly, maintain aircraft trim, maintain rotor rpm and monitor airspeed.

I have logged over 100 full autorotations in a 206 and it glides so beautifully. As far as restarts, I have done that, also.....but in a really nice sim. With the 206, you can only attempt re-start when below 12000 ft PA because you may hot start the engine. It took us about 3,000 ft after the engine was re-started, but it did it. I don't know if I would try that real-world because so much can go wrong!

Hope all that answers your questions.

-Steve

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